Electric discharge device



Dec. V21, 1937. K. H. KINGDON ELECTRIC DISCHARGE DEVICE Filed Feb. 12,1936 l (su Inventor: Kenneth H. Kingdon,

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the operation of the rectifier.

Patented Dec. 2l, 1937 PATENT OFFICE ELECTRIC DISCHARGE DEVICE KennethH. Kingdon, Schenectady, N. Y., assignor to General Electric Company, a.corporation of New York Application February 12, 1936, Serial No. 63,5332 Claims. (VCI. 2250-275) My invention relates to electric dischargedevices adapted to operate in the presence lof an ionizing medium, andmore particularly to such devices as are intended for use with extremelyl high voltages.

In high voltage rectiflers which utilize a vapor or a gas to supplyionization for eliminating space charge there is' a strong tendencyduring the non-conductive period of the rectifier for lo quantities ofsolid material to be thrown 01T the surface of the anode and collectedon the envelope walls. This phenomenon, generally knownas sputtering, isdue to impingement of residual ions driven at high velocity by therelatively in- 16 tense electric eld created in the neighborhood of theanode immediately after the cessation of the conducting period. 'Ihedegree of such sputtering is roughly proportional to the voltagesemployed and becomesvery considerable at--voltages m of the order of 80kilovolts, the walls o f the envelope being noticeably darkened afteroperation lof an hour or so. This darkening is due to the gradualdevelopment of a more or less continuous coating of conducting materialon the inner surg face of the envelope. Since in the ordinary oase thiscoating extends substantially along the entire length of the dischargepath, it has distinctly harmful effects on the breakdown resistance andIn the rst instance it forms a partial short circuit extending fromthepositive to the negative electrode and tends to` concentrate the voltagestress over a much shorter space than that existing between theelectrodes themselves. In actuality the full inverse voltage may beimpressed on the relatively short gaps existing between the mainelectrodes and that portion of the Aenvelope wall which is covered withthe conducting deposit. It' will be obvious that the eect of thiscondition is to de- 40 crease seriously the breakdown strength of thedevice.

Furthermore, the existence of a large metal surface at iloatingpotential in close proximity to the discharge path favors theaccumulation of electrostatic charges of such polarity as to exercise aninhibiting effect on the normal rectier discharge. The result of thispseudo grid eiect may be to postpone the initial conductivity of thedevice considerably beyond the proper point.

It is an object of the present invention to prevent the formation ofcontinuous sputtered coatings capable of producing the harmful effectsmentioned above. For attaining this end I provide shielding meansinterposed between the discharge path and the envelope walls foreffectively intercepting sputtered particles of material and tosubdivide the resultant deposit into a series of spaced and insulatedsections.

The novel features which I believe to be characteristic of my inventionare set forth with 5 particularity in the appended claims. My inventionitself, however, both as to its organization and method of operation,together with other objects and advantages' thereof, will best beunderstood by reference to the following specll0 cation taken inconnection with the accompanying drawing in which Fig. 1 illustrates anelevation in partial section of a discharge device utilizing a preferredembodiment of my invention; Fig. 2 illustrates in partial section analternative i5 modication of which my invention is capable; Fig. 3illustrates a second alternative modification of my invention, and Fig.4 is a partially sectionalized view showing the application of myinvention to a discharge device adapted for use 2o with extremely highvoltage circuits.

Referring particularly to Fig. 1, I have shown a two-electrode rectifiercomprising a sealed envelope of insulating material enclosing anionizable medium such as a small quantity of argon 25 0r mercury. Theenvelope includes a pair of cylindrical sections I and I and is providedat one end with a reentrant stem 2 terminating in a press 3 adapted tobe hermetically sealed to an' electrode lead-in conductor. Such a con-30 ductor 4 is shown as being connected directly to an anode 5 which maycomprise a cup-like member of thoroughly degassed nickel or graphite. Onthat portion of the lead-in conductor between the anode and the pressmay be provided a plu- 35 rality of metallic disks or ballles 6 adaptedto trap random electrons, such as secondary electrons generated from thesurface of the anode, and to prevent their deposit on the portions ofthe stem 2 adjacent the lead-in conductor. In 40 this Way theaccumulation of electrostatic charges suilcient to rupture the envelopewall adjacent the lead-in seal is substantially avoided- At the otherend of the envelope is mounted a second reentrant stem terminating in apress 3' 45 which supports a number of lead-in conductors 1 adapted tosupply heating and load current to a thermionic cathode 8. Although thenature of the cathode is not essential to an understanding of myinvention, it will be understood that it 50 may take the form of ahollow cylinder having therein a number of vane surfaces of extendedarea adapted to be coated with an electron emissive substance, such asan alkaline earth metal. Cathodes of this type are conventionally heated66 by means of a coaxial lament 9 shown in dotted outline. Such acathode is fully described, for example, in Patent No. 1,924,318, A. W.Hull, issued August 29, 1933.

It will be noted that the cylindrical envelope sections I and I ntrather closely around the discharge electrodes. It is apparent that ifsuch an envelope were extended along the entire length of the dischargepath, any high voltage operation of the tube would very quickly resultin the production of a continuous coating of sputtered materialextending between the electrodes. As previously explained, such acoating, because of its close proximity to the electrode tips, wouldseriously weaken the resistance oi.' the tube to undesirable breakdownoccurrences.

In order to avoid this danger in an inexpensive and practical manner, Iprovide at the central portion of the envelope a third cylindricalsection II of greater diameter than the end portions. This is inpartially telescoped relation with the smaller cylinders I and I but isadapted to be joined to them in such a manner as to provide a continuouscompletely closed envelope surface. For this purpose the overlappingedges of the telescoped cylinders are connected and maintained inradially spaced relation by means of serpentine portions I2 and I2having a substantial portion of their length parallel with the main axisof the envelope.

By means of the arrangement described I insure that any deposit ofsputtered material shall be broken up into a numberof spaced sectionsconsisting of portions I3 and I3' collected on the inner extremities ofthe cylinders I and I', and a further segregated portion Il on thecenter of the cylinder Il. Consequently the eective insulating gapbetween the electrodes 5 and cannot be less than twice the distance fromsuch electrodes to the inner surface o! the cylinders I and I plus twicethe distance from such cylinders to the inner wall or the cylinder II.Furthermore adequate creepage distance between the conducting coatingsdeposited on the concentric cylinders is provided by the relativelyextended dimensions oi.' the serpentine portions I2 and I2'.

In Fig. 2 I have shown an alternative mode of application of myinvention In this iigure parts such as the electrodes and lead-inconnections are identically numbered with corresponding parts in Fig. 1.In this embodiment the shielding elements take the form of a series ofrings 20 of insulating material such as glass or quartz having acircular cross section and spaced axially along the central portion ofthe enclosing envelope. This latter is provided with an enlarged section2l to receive the shielding rings and to avoid the possibility of theirinner surfaces approaching too closely to the discharge electrodes. Withthe arrangement illustrated sputtered material particles proceedinglinearly from the region of the discharge path toward the envelope wallwill necessarily fall either on the inwardly exposed surfaces of therings 20 or on those portions or the surface of the cylinder 2| notsubtended by a projection of the rings. Those por tions of the cylinder2l which are shielded by the rings 20 will be maintained substantiallyfree of conducting particles as will those portions of the ringsthemselves which might be visible from outside the envelope. Theultimate result will be that the conducting 'deposit is broken up into aseries of non-contiguous rings suiiiciently insulated from one anotherto reduce substantially the danger of inter-electrode breakdown.r

It is also possible, and lmy invention contemplates, that the shieldingmeans may comprise two or more radially spaced concentric elements. Asillustrating such a modication I have shown in Fig. 3 a discharge deviceresembling those already described in comprising a cylindrical envelope25 enclosing an anode 2B and a cathode 28. In addition an electrostaticcontrol electrode 21 having a lead-in connection 21 is also provided. Inthis case, however, the shielding means consists of a plurality oiradially spaced concentric cylinders 29 and 30 and 29', 30', and 3|',respectively, extending along a portion oi the discharge path. It willbe understood that each of these is open at its ends to permit passageof the discharge current. The lengths of the various cylinders are sorelated that the projections of adjacent cylinders on the envelope wallare of progressively increased area as one proceeds in theoutwarddirection. By this arrangement material removed from the anode by theimpact of ions of the gas or vapor within the envelope is collected andsectionalized by the various shielding cylinders, only a small portionproceeding directly to the outer wall of the envelope.

Due to the longitudinally staggered relation of the individual shieldsthe deposited coatings are positioned to distribute the total potentialdrop as evenly as possible throughout the space between the electrodes.The distribution o! electrostatic stress may be still further improved,however, by providing externally of the envelope one or more metallicrings such as that shown at 32. Such rings should be somewhat spacedfrom the envelope wall to avoid any danger of puncture and may beconnected to a suitable potential to cause a desired redistribution oftbe electrostatic eld.

While I have shown the potential grading ring 32 as being applied in aparticular location, it should be understood that similar rings may bearranged at any region where experience indicates the existence of adangerous iield distortion. If the ring is positioned to overlapslightly the edge of a deposited coating and is maintained at a xedpotential with respect to the cathode, it will serve to cause asuiilcient redistribution of the electrostatic eld to avoid thepossibility of a disruptive breakdown to the sharply terminating edge ofthe coating. A further advantage accruing from the use of externallymounted potential grading rings in connection with external shields ofthe type dealt with by my invention lies in their tendency to preventthe accumulation of positive charges on the cbated surfaces andconsequent interference with the normal tube operation.

In Fig. 4 I have shown the application of my invention to a controllabledischarge device embodying a supplementary electrode for governing thedistribution of the discharge potential along the tube. As illustrated,this comprises an enclosing envelope 35 adapted to contain an ionizablemedium and having at one end thereof an anode 36 consisting or a closedcylinder ofA spun nickel, or similar material. This is mounted on areentrant stem 31 and is provided with a suitable lead-in conductor 38.At the other end of the envelope is provided a thermionic cathode 3lwhich in this instance is shown as consisting ot a simple lamentsupplied with heating current through lead-in conductors 40. Surroundingthe cathode 29 and connected with a suitable external source ofcontrolling potential is arranged a 7l4 tions M grid member Il. This mayconveniently comprise an enlarged cylinder of spun metal having anopening 'at the inner end thereof closed by means ofa mesh screen I2,forexample, of molybdenum.

Since discharge devices of the type with which my invention is concernedare intended `to be used in connection with a source of very highpotential, I have in the present instance provided an intermediateelectrode 43 adapted to be maintained at a potential between that of theanode 36 and that of the cathode 3 9 for the purpose of properlydistributing the electric eld along the length of the discharge device.In accordance with my invention as described in connection with Fig. 1the creation ofa continuous conducting coating on the walls of theenvelope between any two adjacent discharge electrodes is prevented bythe provision of telescoped cylindrical secand 45 whose outercircumferential portions considerably overlap the edges of the smallercylinders which make up the main envelope.

While I have shown a particular embodiment of my invention, it will, ofcourse, be understood that I do not wish to be limited thereto sincemany modiiications in the structure may be made, and I contemplate bythe appended claims to cover all such modifications as fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric discharge device comprising a sealed envelope ofinsulating material enclosing an ionizable medium and spaced electrodes,at least one of which is subject to sputtering, said envelope comprisinga plurality of continuously joined cylinders in partially telescopingrelation, the overlapping edges of said cylinders being radially spacedand positioned to prevent deposit of a continuous coating of sputteredmaterial.

2. In an electric discharge device, a sealed envelope of insulatingmaterial enclosing an ionizable medium and spaced electrodes, at leastone of which is subject to sputtering, said envelope comprising acontinuous outer wall so formed that portions of the Wall directlyopposite the interelectrode space are shielded from the discharge byother portions of the Wall, thereby to prevent deposit of a continuouscoating of sputtered material.

KENNETH H. KINGDON.

